It is well known, especially in the field of telecommunications, to provide electronic circuits on printed circuit boards which are accommodated within an equipment frame, the boards including connectors via which electrical connections are made for example to backplane connectors in the equipment frame. Such an arrangement dictates the size and spacing of the circuit boards.
Evolution of electronic equipment has led to increasingly complex circuits, for example including microprocessors operating at high speeds, to a very high density of components, and to the use of multi-layer circuit boards to accommodate dense conductor patterns, so-called "daughter" boards perpendicular to a main board, and surface mounted components on both sides of the circuit boards. While the use of very large scale integrated circuits has facilitated such evolution, there are still many discrete components, such as transformers, relays, power transistors, large-magnitude capacitors, and precision resistors, which must be accommodated on electronic circuit boards. By way of example, a programmable line interface circuit for telephone lines may incorporate all of these components.
It is almost inevitable that any electronic circuit is subject to revision or improvement after it is initially designed. This is especially the case with circuits, such as telephone line interface circuits, which must be used in a wide variety of differing operating situations and conditions. Often minor revisions or improvements prove to be desirable. While in the long term such changes can be incorporated into a redesign of the physical implementation of the electronic circuit, in the short term this is impractical. Accordingly, such changes have frequently been implemented in the past by electronic "barnacles" on the circuit. An electronic barnacle typically involves the addition of a few components connected into the original design of the circuit.
Electronic barnacles tend to be unsightly and are expensive to implement, because they involve manual modifications of the circuit. While the cost of manually modifying a few circuits may not be great, the costs of manually modifying circuits manufactured in large quantities may be prohibitive.
Furthermore, the high density of components of the circuit, combined with the constraints on its size, may leave virtually no room for components to be added manually to the circuit, because typically the whole area of both sides of the circuit board is fully occupied by components and connections of the original design. Gaining access to points for connecting additional components may also be very difficult or inconvenient. Generally, connection leads of any manually added components must be carefully insulated, involving further manual steps.
Accordingly, a need exists to provide an improved arrangement which facilitates making minor revisions to an electronic circuit design.